1. Field of the Invention
The invention relates to a blood extraction device and more particularly to a blood extraction device having a piston that can be moved in either direction for testing purposes and substantially only in the direction away from the needle when used to extract blood.
2. Description of the Prior Art
Blood extraction devices (see e.g. DE-PS 29 48 653, 30 49 503) are intended merely to extract blood from, but not to inject any kind of fluid substance into the vein of the patient, although they are generally suitable for this purpose.
In conjunction with disposable syringes it is prior art (DE-Gbm 88 04 656) to prevent repeated use by ratchet means allowing only one charging of the syringe and one injection but not a repeat charging with fluid. For this purpose, however, the barrel of the syringe must be provided with a ratchet ring in the form of a piston sliding with friction within the syringe barrel.
In a further disposable syringe of prior art (U.S. Pat. No. 4,826,483) the piston rod is provided with a plurality of ratchet keys which interact with spring-loaded ratchet shoulders at the bottom of the syringe. By turning the piston rod thru 90.degree. the syringe can be charged and then the fluid injected into, for example, the vein of the patient. Since further turning of the piston rod is prevented by the ratchet key locking into place in a radial notch of the piston rod, this syringe too can only be used once for charging with liquid and injecting this liquid.
Also known as prior art is a blood extraction device (U.S. Pat. No. 4,370,987) the piston rod of which is toothed and the rear end of which has an opening for passage of the piston rod and a stop arranged therein. This is intended to establish the position of the piston in various charging positions to enable a vacuum of prescribed strength to be created in the cylinder. As soon as the vacuum is created in the cylinder the rear sharpened end of a needle inserted in the vein of the patient can be used to puncture a flexible plug located in the forward end of the cylinder thus allowing the vacuum to become active within the needle and draw the blood from the vein of the patient.
However, in this blood extraction device of prior art the opening for passage of the piston rod in the rear end of the cylinder must be so large that the toothing on the piston rod and the stop could possibly disengage thus making it possible to push the piston also forwards which is also intended to eject the received blood into some other receiving vessel. This device of prior art could thus also be misused as a syringe.